This invention relates to the field of microwave signal transmission along conductive transmission lines and to energy responsive elements which modify the transmission line characteristics in response to predetermined conditions.
Ionized gas plasma devices have been used in the radar art for changing the characteristics of a microwave transmission line in response to the presence of a high amplitude signal since the early days of radar technology. In a usual embodiment of such apparatus an ionized gas plasma device is employed in conjunction with a one-quarter wave shorted stub, a resonant tuned cavity, a duplexer, an impedance transformer or other articles of microwave "plumbing" for protecting low power sensitive portions of the radar apparatus from large energy signals present in the radar transmitter output. One form of ionized gas plasma device is frequently called a TR tube or an ATR tube and serves to admit large amplitude signals to the radar antenna while blocking such signals from the radar receiver. At a subsequent transmitter off time such devices allow the passing of low amplitude signals from the antenna to the receiver.
In addition to the normal use of TR and ATR ionized gas plasma devices, certain conditions encountered in the radar art make the use of a signal limiting gas plasma device desirable. Conditions calling for use of such a limiting device include the close spacing of more than one radar set as might be accomplished on an aircraft or watercraft where the large amplitude signals from one radar set can be received in the near-by radar set and where the frequency of the two radar sets may be sufficiently different to preclude reliable operation of the normal tuned circuit TR and ATR devices. In addition, the presence of finite firing time delays in most TR and ATR devices call for fast acting limiter devices which can achieve operation before such delays permit damage to sensitive system components.
In recent years the use of microintegrated circuits in the radar art has intensified the need for improved microwave limiting devices that are compatible with the dimensions and technology of the integrated circuit art--compatibility with the planar radio frequency transmission lines employed in the integrated circuit art is especially needed. A significant problem in adapting gas plasma devices to the integrated circuit art is concerned with the arrangement used for priming the gas plasma so as to decrease the time and energy required of the radar transmitter signal before achieving ionization and signal diversion. Existing priming methods such as dc keep-alive devices and radioactive igniters have not been mechanically compatible with a small quartz device containing an active plasma such as might be used in the integrated circuit art. The radio active igniters additionally post a compatability problem with the adjacent integrated circuit, since many of the semiconductor devices used in such circuits are radiation sensitive.
The prior patent art includes several examples of ionized gas devices which are usable in radar circuitry for protection and signal steering purposes. Included in this prior art is the patent of G. L. Tawney, U.S. Pat. No. 2,540,148, which discloses a gas filled TR box that may be employed in a plurality of locations in the signal transmitting paths of a radar set. The Tawney apparatus includes the concept of a resonant cavity, the use of low pressure ionizable gases such as hydrogen and water, and the use of a keep-alive discharge within the resonant cavity of the protective device.
The patent of R. C. Hilliard, U.S. Pat. No. 3,110,832, contemplates the use of a microwave shutter structure for attenuating high energy signals and protecting sensitive microwave receiver components. In the Hilliard apparatus, the shutter structure is combined with a TR tube which employs low pressure gases such as nitrogen, argon or Freon.RTM. that are operated in the presence of a keep-alive discharge. The name Freon.RTM. is a registered trademark of E.I. DuPont de Nemours and Company.
The patent of D. C. Broderick, U.S. Pat. No. 3,521,197, discloses a power limiter device for a waveguide wherein a gas filled quartz cylinder of precise dimensions is mounted between two waveguide apertures at a point of maximum voltage in a standing-wave pattern. The Broderick device contemplates use of both a gas filled tube and a semiconductor or varactor diode. The Broderick patent also discloses alternate embodiments for a power limiter in which a series of ionization gaps sealed in a low pressure environment together with a keep-alive electrode are employed. In yet another embodiment, Broderick discloses the combination of a power limiter and an ionizable gas TR tube with the limiter section being comprised of solid state elements.
Two of my own prior patents, each involving a coinventer and one assigned to my present employer and one to the Government of the United States concern the use of a yttrium-ion garnet (YIG) spherical element for performing a power limiting and signal filtering function. The first of these patents, U.S. Pat. No. 4,155,053 is concerned with improving the coupling between the signal transmission path and the YIG element through the use of a groove in the transmission line structure while the second, U.S. Pat. No. 4,155,054 is concerned with mounting the YIG element within the microwave slotted resonant structure.
A third of my prior patents, U.S. Pat. No. 4,277,437, also involving a coinventor and assignment to the Government of the United States is concerned with a TR switch structure which is fabricated from boron nitride and filled with a low pressure halogen gas such as chlorine. The U.S. Pat. No. 4,277,437 provides for improving the thermal conductivity by which energy is removed from the ionized gas of the TR tube and improving the susceptibility of the TR switch to the gas cleanup mode of failure.
A fourth of my prior patents, also involving coinventors and an assignment to the Government of the United States, U.S. Pat. No. 4,193,047, discloses a microwave power limiter which employs a dielectric resonator structure having a low threshold power, a large dynamic range, and mechanical and temperature stability. The U.S. Pat. No. 4,193,047 contemplates the use of a YIG dissipating element biased by a magnetic field and energized by the radio frequency magnetic field existing within a waveguide structure.
A fifth of my prior patents, U.S. Pat. No. 4,395,684 and a related technical article "An RF-Primed All-Halogen Gas Plasma Microwave High-Power Receiver Protector" appearing in the Institute of Electrical and Electornic Engineers Transactions on Microwave Theory and Techniques, Volume MTT-30, Number 12, December 1982, at pages 2177-2184, concern the concept of using radio frequency energy for achieving gas priming in a plasma limiter device. Both of these documents originate with myself and a colleague and each includes an interest by the government of the United States. As described in these documents, radio frequency energy can be advantageously employed to replace or supplement the radioactive and DC keep-alive discharge priming arrangements previously used in microwave limiter devices.
This use of radio frequency priming in plasma limiter devices provides a notably improved arrangement wherein physical contact between the active limiter media such as an ionized gas and metallic elements in the microwave structure can be completely avoided. This improvement is based on sealing of the limiter media within a completely inert vessel such as a quartz capillary tube. With this sealed isolation the desired media standby ionization is achieved by radio frequency electromagnetic field coupling into the media capillary rather than with the previous arrangement of electrodes passing through the capillary walls or sealing a radioactive material within the media container. These previous arrangements have several disadvantages, including the promotion of contamination (e.g. gas cleanup) of both the media material and the ionization producing element.
As is indicated in the December 1982 article, the ability to employ RF primed limiter devices in all stages of a receiver protector limiter device also combines desirably with the characteristics of certain PIN diodes, allows use of aluminum or other desirable metals in the microwave components, allows great freedom in selecting the gases employed in the limiter media and provides other advantages over previous arrangements. An especially notable of these other advantages is the enabling of extremely fast ionization recovery periods in the limiter gas media in comparison with prior arrangements. This improvement enables improved radar range and pulse repetition frequency performance, for example. This improvement is moreover principally achieved through the increased freedom available in selecing media gas composition when gas to metal contact is removed as a consideration.
The present invention contributes to the desirability of this isolated media arrangement in limiter devices by enabling the achievement of a reliable and stable priming discharge in the isolated media--a discharge that is immune to frequency drift in the priming energy source as inherently tends to occur when microwave equipment is exposed to military temperature extremes, for example.